Among a wide variety of thermoplastic resins developed in recent years, those called engineering plastics including polyamide, polyester, polycarbonate, polyacetal, polysulfone, polysilicone, polyphenylene oxide, polyimide, ABS, and methacrylate resins generally exhibit excellent performances, such as mechanical strength, heat resistance, creep resistance, chemical resistance, electrical characteristics, dimensional stability, and the like. Many of the engineering plastics can be used under a broad range of conditions in substitution for metals, e.g., iron, zinc, aluminum, etc. For example, polyamide resins have been applied to gasoline tanks.
Some of molded articles of the engineering plastics cannot be produced by means of a single mold. For example, composite laminates comprising a metallic insert, a resin molded part having inserted therein a metallic insert, or a plastic insert (e.g., female screws or coils of complicated shape) and an injection-molded part, such as a composite article comprising a resin pipe and a resin joint, can be produced by setting the insert part in a cavity of an injection mold and then injecting a molten resin into the cavity to unite the insert part and the injected resin into one body.
In order to obtain satisfactory bonding strength between the insert part and the injection-molded part, the insert part is coated previously with a liquid adhesive or a primer, or a surface of a vulcanized rubbery insert part is treated with trichlene or trichloroisocyanuric acid prior to injection molding.
The liquid adhesive or primer to be used include solvent-based polyester resins, liquid-type epoxy resins, ethyene-methacrylic acid copolymer metal salts (e.g., Zn.sup.+, Na.sup.+ and K.sup.+ salts), maleic anhydride-grafted ethylene-vinyl acetate copolymers, styrene-butadiene copolymer latex, polyester aqueous emulsions, polyvinyl acetate aqueous emulsions, liquid phenolic resins, etc.
However, in molded articles having an insert part in which a metallic part is further incorporated as an integral part, molded articles having a complicated shape, or molded articles required to have resistance to pressure or thermal shock, it has recently been pointed out that insufficient laminate bonding strength between the insert part and the injection-molded part causes problems such as leakage of a liquid content from the molded article or destruction of the molded article, even when the insert part and the injected resin are the same in kind or even when an adhesive or primer is applied between the insert part and the injected resin which are different in kind. This problem is particularly acute in small-sized products or products requiring accuracy.
Taking an example of a laminated container composed of a metallic insert part and an engaging plastic injection molded part such as polyacetal, polyamide, polybutylene terephthalate, etc., when the entire inner wall of the metallic plate is coated with engineering plastic as in a resin-coated steel container, some degree of laminate bonding strength would be enough for use as a tank for gasoline, motor oil, petroleum, water, etc.
However, in cases where only a part of a metallic part is covered with engineering plastic as exemplified by an automobile part as illustrated in FIG. 7, when the part is immersed in an liquid, e.g., gasoline, motor oil, water, etc., or dried, or repeatedly receives thermal shock, the liquid tends to enter into the boundary between metallic part 4 and the engineering plastic injection molded part 9 to reduce bonding strength therebetween, which would ultimately result in leakage of the liquid from the laminate. The leakage is more apt to occur on application of pressure (e.g., 0.5 to 3 kg/cm.sup.2) to the liquid. This is because the coefficient of thermal expansion of the metal is smaller than that of the engineering plastic so that the resin layer comes to release through repetition of temperature changes between -20.degree. C. and +80.degree. C. (i.e., thermal shock). Depending on the kind of the resin, leakage of the liquid content sometimes occur due to dissolution of the resin in gasoline or motor oil.
Adhesives or primers having a glass transition point of not higher than -20.degree. C. and providing a film having an elongation of 1500% or more, such as an acrylonitrile-butadiene copolymer aqueous emulsion (Tg: -25.degree. C.) or a styrene-butadiene copolymer latex might be used to form an adhesive film which can follow dimensional changes caused by temperature changes. These adhesives, however, still fail to exhibit sufficient resistance to thermal shock or chemicals and undergo reduction in bonding strength to cause leakage of gasoline or water.
According to the state-of-the-art techniques, the problem of leakage is coped with by coating the insert with an epoxy resin or a vulcanized organopolysiloxane, but such a coating becomes useless in a short duration through repetition of temperature changes, electrical shocks or voltage application to electrical precision parts or pressure application (0.5 to 3 kg/cm.sup.2) to motor oil, water, etc. Therefore, release takes place on the adherend surface or coated surface to cause liquid leakage, errors in operation, or failures in machinary.